Schedule of Cash, Cash Equivalents and Available-for-Sale Securities by Significant Investment Category (Detail) - USD ($) $ in Thousands		Sep. 30, 2018	Dec. 31, 2017		Sep. 30, 2017	Dec. 31, 2016
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		$ 1,651,566	$ 1,786,045
Gross Unrealized Gains		279	300
Gross Unrealized Losses		(3,586)	(4,592)
Fair Value		1,648,259	1,781,753
Cash and Cash Equivalents		882,184	598,028	[1]	$ 431,399	$ 408,330
Short-term Marketable Securities	[2]	561,192	797,940	[1]
Long-term Marketable Securities	[3]	204,883	385,785	[1]
Level 1 | Cash
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Cash		208,562	340,253
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Cash and Cash Equivalents		208,562	340,253
Short-term Marketable Securities	[2]	0	0
Long-term Marketable Securities	[3]	0	0
Level 2
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		1,443,004	1,445,792
Gross Unrealized Gains		279	300
Gross Unrealized Losses		(3,586)	(4,592)
Fair Value		1,439,697	1,441,500
Cash and Cash Equivalents		673,622	257,775
Short-term Marketable Securities	[2]	561,192	797,940
Long-term Marketable Securities	[3]	204,883	385,785
Level 2 | Money Market Instruments
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		657,334	215,441
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		657,334	215,441
Cash and Cash Equivalents		657,334	215,441
Short-term Marketable Securities	[2]	0	0
Long-term Marketable Securities	[3]	0	0
Level 2 | Corporate Debt Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		506,054	707,652
Gross Unrealized Gains		133	150
Gross Unrealized Losses		(2,498)	(2,553)
Fair Value		503,689	705,249
Cash and Cash Equivalents		0	3,096
Short-term Marketable Securities	[2]	350,109	406,188
Long-term Marketable Securities	[3]	153,580	295,965
Level 2 | Commercial Paper
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		70,784	24,566
Gross Unrealized Gains		0	0
Gross Unrealized Losses		0	0
Fair Value		70,784	24,566
Cash and Cash Equivalents		16,288	2,751
Short-term Marketable Securities	[2]	54,496	21,815
Long-term Marketable Securities	[3]	0	0
Level 2 | U.S. Government Agency Securities
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		201,706	472,593
Gross Unrealized Gains		0	0
Gross Unrealized Losses		(1,087)	(1,975)
Fair Value		200,619	470,618
Cash and Cash Equivalents		0	35,497
Short-term Marketable Securities	[2]	149,512	345,501
Long-term Marketable Securities	[3]	51,107	89,620
Level 2 | Foreign and Other
Schedule Of Cash Cash Equivalents And Available For Sale Securities [Line Items]
Amortized Cost		7,126	25,540
Gross Unrealized Gains		146	150
Gross Unrealized Losses		(1)	(64)
Fair Value		7,271	25,626
Cash and Cash Equivalents		0	990
Short-term Marketable Securities	[2]	7,075	24,436
Long-term Marketable Securities	[3]	$ 196	$ 200

[1] December 31, 2017 balances were derived from the audited Consolidated Financial Statements included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2017, filed with the SEC on February 26, 2018.

[2] The Company’s short-term marketable securities mature in one year or less.

[3] The Company’s long-term marketable securities mature between one and five years.